Content knowledge and pedagogical practices are of particular concern to middle school science instructors teaching density. First introduced in elementary grades with the ideas of floating and sinking, density taught in middle school is geared toward understanding through the use of mathematical formulas. Using a lesson-plan study design, the authors identified preservice teachers' content understandings regarding density and how those understandings influenced their teaching practices. The results indicated that, although teacher candidates had developed reasonable, intuitive understandings of density, many experienced difficulty connecting those understandings to the mathematical relationships involved. Therefore, university science educators must carefully examine the levels at which their students understand density so that these future educators have the necessary conceptual knowledge to teach this concept effectively. (Contains 1 table and 1 figure.)